The best way to do avionics is to get it new or refurb from the USA e.g.
Southeast Aerospace and then find a friendly avionics man to install it on a daily charging rate.
The Q whether a GPS will do for a DME has never been settled. The normal wording in the equipment carriage regs is "distance measuring equipment" and it doesn't say it has to use the DME beacon. IMHO using a GPS instead of a DME is stretching the bounds of imagination but I am not a lawyer. I don't know of any prosecutions (UK or Europe) under the equipment carriage regs, but I don't think that is ever the problem in aviation - the real issue is insurance not paying out on a flight which was illegal to start with.
A more practical view is what kind of kit you want to backup a GPS. I would have a VOR/DME. ADF is not so good but it remains essential due to the large # of NDB approaches in Europe; again one actually flies them with GPS but checking the ADF at the FAF is a good idea.